Large wind turbine systems for offshore or onshore use convert rotational energy of a rotor driven by the wind into electrical energy. The conversion is either done directly or indirectly. In the first case the relatively slow rotation of a drive train connected to the hub of the wind turbine is directly converted into electrical energy. This is realized by directly attaching a rotor of a generator to the drive train, while a stator of this generator is assembled around the drive train in the region of the generator's rotor. Together, the stator and the rotor attached to the drive train form the main parts of the generator. It may be noted in this context, that in any of the following embodiments, the rotor (and this may imply the drive train) can also be assembled around a stator just as well. In contrast, indirect conversion means that the relatively slow rotation of the drive train is converted in a gear box into a faster rotation speed of a second drive train (named “output drive train” in the following passages) which then feeds a generator.
Direct driven generators are generally much bigger, heavier and thus costlier to produce and assemble. However, while gear box driven generators are lighter, smaller and thus cheaper they have the disadvantage of necessitating an additional part, i.e. the gear box. Such gear box requires additional assembly and maintenance efforts and is yet again an expensive component. In addition, the noise emission of such gear box in comparison with a direct drive generator is also an important factor which needs to be considered.
Gear box-based generators can be realized as a so-called multibrid and/or multi-generator assemblies. A multibrid system means that the gear box and the generator are included in one single housing. Additionally, such systems may also comprise an azimuth drive (for orientating the wind turbine into the wind). Optionally the shaft bearing of the wind turbine's main shaft may also be integrated in the housing.
A multiple generator assembly means that several generators are assembled behind a gear box which converts the rotational energy of the drive train into rotations of several second output drive trains which then lead into one generator each. The system has the advantage of allowing the use of even smaller generators instead of one larger generator and of a high efficiency factor.
Still, even with multibrid or multiple-generator assemblies there remains the problem of relatively high assembly and maintenance efforts for indirectly-driven generator systems.